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Abstract. A series of titanium complexes bearing substituted diphenolate ligands (RCH(phenolate)2, where R = H, CH3, o-OTs-phenyl, o-F-phenyl, o-OMe-phenyl, 2,4-OMe-phenyl) was synthesized and studied as catalysts for the ring opening polymerization of L-lactide and s-caprolactone. Ligands were designed to probe the role of chelate effect and steric effect in the catalytic performance. From the structure of triphenolate (with one extra coordination site than diphenolate ligand) Ti complex, TriOTiOiPr2, we found no additional chelation to influence the catalytic activity of Ti complexes. It was found that bulky aryl groups in the diphenolate ligands decreased the rate of polymerization most. We conclude that steric effect is the most controlling factor in these polymerization reactions by using Ti complexes bearing diphenolate ligands as catalysts since it is responsible for the exclusion of needed space for incoming monomer by the bulky substituents on the catalyst.
Keywords: biodegradable polymers, polycaprolactone, ring-opening polymerization, polylactide
1.Introduction
Ring-opening polymerization (ROP) is the most common method to synthesize biodegradable polyesters, such as poly(lactide) (PLA) and poly(s-caprolactone) (PCL) [1, 2]. These polymers have vast applications in many fields [3, 4]. Metal catalysts [5-7] have been commonly used as catalysts for the ROP of cyclic esters. Since PLA and PCL polymers have wide application in medical field, the minimization of residual hazardous metal in these polymers after synthesis is essential. For this reason as well as its low cost, titanium complexes [8-14] were chosen as popular catalysts in ROP. Among these titanium complexes, multi-phenol ligands have proven particularly useful because of its ease of synthesis and diverse structural variation [11-14].
Two main features of Ti diphenolate complexes that influence their catalytic performance were reported in the literature-chelate effect and steric effect (Figure 1). Chelate effect exerted by extra donor coordination on diphenolate ligand on Ti catalyst was found to enhance their catalytic performance. This effect was investigated by Takashima et al. [12] through the application of titanium catalysts bearing chalcogenbridged diphenolate ligands in s-caprolactone (CL) polymerization. Their results demonstrated that extra sulfur donor was more effective with regard to initiation. In a separate study, Takashima et al. [13] also found that the tellurium-bridged titanium complex was superior to the sulfur-bridged titanium complex catalytically. It is clear that...